Porcine reproductive and respiratory syndrome (PRRS) is characterized by severe reproductive failure and a high rate of late abortion and early farrowing in sows, and respiratory disease and mortality in young pigs. PRRS is caused by a small, enveloped virus with a single-stranded positive-sense RNA genome, which belongs to the family Arteriviridae, genus Arterivirus. PRRS virus naturally replicates in alveolar macrophages, and is able to maintain a prolonged viremia, causing persistent infections that last for months in some instances. The disease suddenly emerged in the late 1980s in the US and Europe, and has since spread worldwide, causing major economic losses to the swine industry. The virus is able to persist on infected farms, mainly due to its presence in persistently infected carrier sows.
PRRS virus is classified in two genotypes based on its continent of origin. PRRS virus strains originating from North America are classified as type 2 genotype, while those originating from Europe are designated as type 1 genotype. Currently, both genotypes circulate globally. The two genotypes differ approximately 40% from each other at the genomic level and are also serologically distinct. Isolates within each genotype also exhibit considerable nucleotide sequence heterogeneity of up to 20%. PRRS virus appears to evolve by random mutation and intragenic recombination events.
Based on sequence analysis of Spanish strains, it has been estimated that PRRS virus exhibits a mutation rate of 1 to 3×10−2 substitutions per site and year, which is similar to that of other rapidly evolving RNA viruses. The immense genetic variation of PRRS virus that has been observed over that last 25 years and the appearance in the field of PRRS virus isolates producing much higher morbidity and mortality than earlier isolates is remarkable. In addition, the fact that each stock of PRRS virus typically exists as a mixture of genetically related species is becoming increasingly recognized.
A common type of biologic used in veterinary medicine to protect animals from viral diseases consists of modified live virus (MLV) vaccines. The most frequently used method for producing an attenuated live virus vaccine is to serially passage the pathogenic virus in a substrate (usually cell culture) other than the natural host cell and/or in adverse conditions until it becomes sufficiently attenuated from its original virulence (disease-producing ability), but retains its ability to induce protective immunity. In 1996 the first MLV vaccine was introduced into the North American market and was based on the PRRS virus strain VR-2332 isolated in 1991. The attenuated vaccine strain was derived by 25 serial passages of this virus at 35-37° C. in simian kidney cells (MA-104/MARC-145) followed by 12 additional passages at 31° C. in the same type of cells, for a total of 36 passages.
Subsequently, in response to a perceived decrease in the protective efficacy of the original PRRS MLV vaccine, presumably due to evolving genetic changes in the genome of prevalent PRRS virus isolates, which resulted in the emergence of more virulent and genetically dissimilar (heterologous) strains of PRRS virus, a second version of an MLV vaccine was introduced in 1999. The rationale for this initiative was to increase the genetic homology of the vaccine strain over that of the contemporary viruses circulating in the field in the late 1990s. This attenuated vaccine strain was derived from the JA-142 PRRS virus isolated from a severe case of PRRS in 1997 and represented the 200th serial passage of this isolate at 37° C. in the monkey kidney cell line MARC-145. The two progenitor isolates for these vaccines, VR-2332 and JA-142, have been described to exhibit moderate and high levels of virulence, respectively, thus explaining the need for either a moderate number of passages under adverse conditions (VR-2332) or a much greater number of serial passages in a milder environment (JA-142) in cell culture in order to generate an attenuated vaccine virus. Notably, inoculation of these attenuated PRRS virus strains into swine results in a viremia lasting more than 4 weeks. During this time the virus is shed in body secretions, resulting in the transmission of the vaccine virus to unvaccinated animals. As a result, the use of these vaccines has led to their reversion from an attenuated to a virulent phenotype.
Infection of pigs with wild type PRRS virus or their vaccination with a live attenuated form of this pathogen elicits production of virus-specific but non-neutralizing antibodies and a meager production of neutralizing antibodies. In addition, during this time, limited quantities of interferon (IFN) gamma secreting cells (SC) are generated. Production of virus-neutralizing antibodies as well as virus-specific IFN gamma SC are considered to be the main determinants for eliciting protective immunity against PRRS virus. It is well accepted that PRRS virus inherently stimulates imbalanced (i.e., a strong humoral response characterized by abundant production of non-neutralizing antibodies and a limited, but potentially protective, T cell-mediated, IFN gamma-based cellular immunity) and non-protective immune responses. It had been previously proposed that the most relevant parameter determining development of the often-observed non-protective adaptive immune response to vaccination or infection is the lack of an adequate innate immune response elicited by PRRS virus. Usually, virus-infected cells secrete type I IFN (IFN alpha and IFN beta), which elicits molecular changes in the neighboring cells to help them protect themselves from virus infection. Notably, the IFN alpha response of pigs to infection with PRRS virus is nearly non-existent.
It has been postulated that the absence of an adequate innate immune response to infection or vaccination with PRRS virus could be at least partly responsible for the belated production of specific virus-neutralizing antibodies and the protracted development of a cell-mediated immune response of pigs against this virus. Thus, PRRS virus may circumvent the genesis of a Th-1 type response by not eliciting adequate IFN alpha production upon infection of its host. In this regard, it is known that plasmacytoid dendritic cells (pDC) play a central role in the induction of an early antiviral state due to their prompt and copious secretion of IFN alpha in addition to other cytokines, e.g. tumor necrosis factor (TNF) alpha and interleukin 6 (IL-6), that have a significant impact on the development of adaptive immunity. Even though pDC represent only a small fraction (<1%) of the porcine peripheral blood mononuclear cell (PBMC) population, they account for the majority of secreted IFN alpha in freshly isolated porcine PBMC samples. Notably, unlike other porcine viruses that stimulate pDC to secrete abundant amounts of IFN alpha, PRRS virus elicits a meager IFN alpha response by this cell subset, and even negatively affects their function by actively suppressing the ability of stimulated pDCs to secrete IFN alpha and TNF alpha. Such obstruction could be reasonably expected to have a significant impact on the nature of the host's subsequent adaptive immune response. Support for this hypothesis was provided by the enhancing effect that providing an exogenous source of IFN alpha at the time of immunization with a PRRS MLV vaccine had on the intensity of the PRRS virus-specific, T cell mediated IFN gamma response.
There is a long felt need in the art for an effective and economical vaccine to protect swine from the effects of PRRS infection so that losses will be minimized.